In LTE-Advanced, an improved version of 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), hybrid transmission, in which switching is performed between SC-FDMA (Single Carrier-Frequency Division Multiple Access) and OFDMA (Orthogonal Frequency Division Multiple Access) is performed in an uplink, has been investigated (see Non-Patent Literature 1, for example).
An advantage of OFDMA is that more flexible frequency resource allocation is possible than in the case of SC-TDMA, and therefore frequency scheduling gain is obtained. Thus, OFDMA enables throughput performance to be improved. On the other hand, an advantage of SC-FDMA is that PAPR (Peak-to-Average Power Ratio) denoting a ratio of peak to average power of a transmission signal, and CM (Cubic Metric), are smaller than in the case of OFDMA. Consequently, if power amplifiers with the same maximum transmission power specification are used for SC-FDMA and OFDMA, power amplifier back-off necessary for transmitting a transmission signal without distortion can be made smaller in the case of SC-FDMA. Thus, SC-FDMA can increase actually transmissible maximum power, enabling coverage performance to be improved.
Hybrid transmission enables the respective above advantages to be obtained by switching adaptively between SC-FDMA and OFDMA according to the communication environment of a mobile station.
Investigation has been carried out into having control of switching between SC-FDMA and OFDMA performed by a base station based on power headroom (hereinafter referred to as “PHR”) information indicating a margin of power (possible increase in power) of the transmission power of a mobile station. Non-Patent Literature 1 describes applying OFDMA to a mobile station with a PHR margin because transmission power is low, and applying SC-FDMA to a mobile station with no PHR margin because transmission power is high.
The PHR definition and transmitting method investigated in LTE will now be described. With LTE, a mobile station transmits PHR by means of a data channel in order for PHR to be used when a base station performs transmission power control, MCS (Modulation and channel Coding Scheme) control, and transmission bandwidth control. Non-Patent Literature 2 includes a PHR definition and PHR transmission conditions according to equation 1.PHR=10 log10(PMAX)−(10 log10 M+P0+αPL+ΔMCS+ƒ(Δi))  (Equation 1)
Here, PHR denotes power headroom [dB], PMAX denotes maximum transmission power [mW], M denotes an allocated number of frequency resource blocks, P0 denotes an offset (a parameter signaled from a base station) [dB], PL denotes a path loss level [dB], α denotes a weighting coefficient for path loss, ΔMCS denotes an MCS-dependent offset, and f(Δi) denotes a transmission power control value subject to closed-loop control.
When a mobile station moves, path loss fluctuates, and therefore PHR fluctuates temporally. Consequently, it is necessary for a mobile station to report PHR to a base station at a predetermined period or when a predetermined condition is satisfied. Non-Patent Literature 2 discloses reporting of PHR to a base station by a mobile station if PHR is Y [dB] or below or if path loss changes by X [dB],
and also describes reporting of PHR at N-frame intervals (where Y, X, and N are parameters).